U.S. patent number 5,939,641 [Application Number 09/127,707] was granted by the patent office on 1999-08-17 for system and method for empirically determining shrinkage stresses in a molded package and power module employing the same.
This patent grant is currently assigned to Lucent Technologies Inc.. Invention is credited to Ashraf W. Lotfi, John D. Weld.
United States Patent |
5,939,641 |
Lotfi , et al. |
August 17, 1999 |
System and method for empirically determining shrinkage stresses in
a molded package and power module employing the same
Abstract
A system for, and method of, empirically determining stress in a
molded package and a power module embodying the system or the
method. In one embodiment, the system includes: (1) a sensor,
having a magnetic core exhibiting a known complex permeability in a
control environment, that is embedded within the molded package and
therefore subject to the stress and (2) a measurement circuit,
coupled to the sensor, that applies a drive signal to the sensor,
measures a response signal received from the sensor and uses the
drive signal and the response signal to determine a complex
permeability under stress of the core. The magnitude of the stress
can then be determined from the core's complex permeability under
stress.
Inventors: |
Lotfi; Ashraf W. (Rowlett,
TX), Weld; John D. (Succasunna, NJ) |
Assignee: |
Lucent Technologies Inc.
(Murray Hill, NJ)
|
Family
ID: |
25471682 |
Appl.
No.: |
09/127,707 |
Filed: |
July 31, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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938619 |
Sep 25, 1997 |
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Current U.S.
Class: |
73/768;
73/779 |
Current CPC
Class: |
G01B
7/24 (20130101) |
Current International
Class: |
G01B
7/16 (20060101); G01B 7/24 (20060101); G01B
007/16 () |
Field of
Search: |
;73/763,768,771,774,779,789 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
US. Patent Application entitled "Encapsulated Package for Power
Magnetic Devices and Method of Manufacture Therefor" Authors: A.W.
Lotfi. J.D. Weld. K.E. Wolf and W.L. Woods: Filed on Feb. 21, 1996:
U.S. Serial No. 08/604.637..
|
Primary Examiner: Noori; Max H.
Parent Case Text
This application is a divisional of application Ser. No.
08/938,619, filed Sep. 25, 1997, currently pending. The
above-listed application Ser. No. 08/938,619 is commonly assigned
with the present invention and is incorporated herein by reference.
Claims
What is claimed is:
1. A power module, comprising:
a power train having:
a conversion stage that includes a power switching device for
receiving input electrical power and producing therefrom switched
electrical power,
a power transformer, coupled to said conversion stage, that
includes a core, a primary winding and a secondary winding,
a rectifier, coupled to said power transformer, including
rectifying diodes, and
a filter stage, coupled to said rectifier, that includes an output
inductor and an output capacitor for filtering said switched
electrical power to produce output electrical power;
a molding compound that embeds said power train to form a molded
package; and
a stress determination circuit for empirically determining stress
in said molded package, including:
a sensor, having a magnetic core exhibiting a known complex
permeability in a control environment, that is embedded within said
molded package and therefore subject to said stress, and
a measurement circuit, coupled to said sensor, that applies a drive
signal to said sensor, measures a response signal from said sensor
and uses said drive signal and said response signal to determine a
complex permeability under stress of said core and a magnitude of
said stress therefrom.
2. The power module as recited in claim 1 wherein said sensor
further has drive and sense windings located proximate said core,
said drive winding receiving said drive signal and said sense
winding generating said response signal.
3. The power module as recited in claim 1 wherein said sensor is
integrated into said power train.
4. The power module as recited in claim 3 wherein said sensor is
embodied in said power transformer.
5. The power module as recited in claim 1 wherein said core is
composed of a ferrite.
6. The power module as recited in claim 1 wherein said measurement
circuit is located in said molded package.
7. The power module as recited in claim 1 wherein said sensor and
measurement circuit are operable during a molding of said molded
package.
Description
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to power magnetics
and, more specifically, to a system and method for empirically
determining stresses (including shrinkage stresses) in a molded
package (that may contain electronic components or magnetic
devices) and a power module employing the system or the method.
BACKGROUND OF THE INVENTION
Magnetostriction is a phenomenon that occurs in magnetic materials
such as ferrites, metals and alloys. Magnetization of the materials
causes a change dl in a dimension 1, creating a strain or
magnetostriction .lambda. represented by .lambda.=dl/1.
Magnetostriction is a material constant that can be either negative
or positive. A magnetic material exists in one of three states or
regimes. Above a Curie temperature, the material is in a
paramagnetic regime and exhibits no magnetization. Below the Curie
temperature, the material may be in either a ferromagnetic regime
or a saturation magnetization regime. In the ferromagnetic regime,
spontaneous magnetization occurs in small, randomly ordered
molecular magnetic domains throughout the material. The overall
magnetization of the material is, however, zero. A strong magnetic
field, sufficient to align all the molecular magnetic domains, may
be applied to the material to place it in a saturation
magnetization regime. In this regime, the alignment of the
molecular magnetic domains produces a maximum length change in the
material and provides a value for the saturation magnetostriction
of the material.
Power modules are employed in many electronic devices to power the
components therein. Power modules were initially available in
through-hole packages consisting of a metal or plastic case,
housing a printed wiring board (PWB) on which power module
components were mounted.
Electronic components are currently migrating towards surface-mount
packaging in overwhelming proportions. Board-mounted power modules
will inevitably follow, if only to assure assembly compatibility
with this packaging technology. Surface-mount assembly operations,
however, typically involve severe reflow temperatures and wash (or
cleaning) cycles that may damage components in the power modules.
As a result, power module circuits are encapsulated in a rigid
epoxy molding compound via, in most cases, a transfer molding
process. As a dense glass-filled epoxy with a high glass transition
temperature and a high modulus, the molding compound is capable of
withstanding the high temperatures found in surface-mount assembly
operations. During encapsulation, the molding compound completely
fills around all the components in the power module circuits,
creating a solid package for the power module and providing a good
thermal path for heat generating components. The molding compound
thus protects the power module circuits from surface-mount assembly
operations.
The protection provided by the molding compound comes, however, at
a cost. As the molding compound cools from a molding temperature to
room temperature, it shrinks. Substantial thermal shrinkage
stresses are thus imposed on the components in the power modules by
the high modulus molding compound.
Power modules typically use ferrite materials [(e.g., manganese
zinc (MnZn)) ] as core materials in magnetic devices such as power
transformers and energy storage inductors. As the molding compound
shrinks and thermal shrinkage stresses are imposed on the ferrite
materials, large strains are created, restricting the movements of
the small, molecular magnetic domains during external magnetic
field excitation. The required degree of alignment of the molecular
magnetic domains cannot be achieved.
Strain pinning between the domain walls occurs, increasing
dissipation in the ferrite materials. The ferrite materials,
therefore, cannot fully enter the saturation regime. A pronounced
decrease in the magnetic properties results, with a corresponding
degradation in performance of the magnetic devices (e.g., power
transformers and energy storage inductors). As described in U.S.
Ser. No. 08/604,637 filed on Feb. 21, 1996, "Encapsulated Package
for Power Magnetic Devices and Method of Manufacture Therefor,"
magnetic devices containing ferrite materials, therefore, must be
protected from the thermal shrinkage stresses of the molding
compound to retain full functionality in surface-mount power
modules or power modules in general.
Since the transfer molding process is widely used in packaging
integrated circuits, a determination of molding stresses in the
packages during molding is necessary to the design of a long-life
and robust package. Precise knowledge of thermal shrinkage stresses
is also necessary in the development of protection schemes for the
ferrite materials used in power modules.
Knowledge of thermal shrinkage stresses in a molded package is
typically obtained through analytical models and determined by
conventional stress analysis or finite element analysis. This
knowledge, however, is only theoretical. It would be advantageous
therefore, to measure the shrinkage stresses during the three
stages of molding (i.e., filling, packing and cooling).
Accordingly, what is needed in the art is a method of empirically
determining the stresses (including shrinkage stresses) present in
a molded package.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the
present invention provides a system for, and method of, empirically
determining stress in a molded package and a power module embodying
the system or the method. In one embodiment, the system includes:
(1) a sensor, having a magnetic core exhibiting a known complex
permeability in a control environment, that is embedded within the
molded package and therefore subject to the stress and (2) a
measurement circuit, coupled to the sensor, that applies a drive
signal to the sensor, measures a response signal received from the
sensor and uses the drive signal and the response signal to
determine a complex permeability under stress of the core and a
magnitude of the stress therefrom.
The present invention therefore introduces the broad concept of
embedding a sensor in a molded package to empirically determine the
amount of stress present in the package. Feedback from stress
measurements allows component designs (e.g., inductor parameters
such as protection, gap, turns, etc.) to be adjusted to compensate
for magnetostrictive effects. Once modifications are made during
product development and design, regular stress measurements made
during manufacturing will allow monitoring of the molded product.
Excessive stresses leading to a failed component may thus be
detected at the molding stage rather than at a later stage after
other value-added operations have been performed.
In one embodiment of the present invention, the sensor further has
drive and sense windings located proximate the core, the drive
winding receiving the drive signal and the sense winding generating
the response signal. Alternatively, the sensor may have only one
winding, in which case self-induction, as influenced by the
permeability of the core, produces the response signal.
In one embodiment of the present invention, the molded package
contains a power module and the sensor is integrated into a power
train of the power module. In a more specific embodiment, the
sensor is selected from the group consisting of: (1) a transformer
in the power module and (2) an inductor in the power module. Thus,
the present invention may be separate from other circuitry embedded
in the molded package or may employ a magnetic device preexisting
in the circuitry to perform stress determination.
In one embodiment of the present invention, the core is composed of
a ferrite. In a more specific embodiment, the core is composed of
manganese zinc (MnZn) ferrite. Those skilled in the art will
understand, however, that the present invention is operable with
all materials that are subject to magnetostriction.
In one embodiment of the present invention, the measurement circuit
is located in the molded package. Of course, the measurement
circuit can be located outside of the molded package.
In one embodiment of the present invention, the sensor and
measurement circuit are operable during a molding of the molded
package. This allows stress to be measured throughout the molding
process, providing valuable insight into optimal production
techniques. However, such capability is not necessary to the broad
scope of the present invention.
The foregoing has outlined, rather broadly, preferred and
alternative features of the present invention so that those skilled
in the art may better understand the detailed description of the
invention that follows. Additional features of the invention will
be described hereinafter that form the subject of the claims of the
invention. Those skilled in the art should appreciate that they can
readily use the disclosed conception and specific embodiment as a
basis for designing or modifying other structures for carrying out
the same purposes of the present invention. Those skilled in the
art should also realize that such equivalent constructions do not
depart from the spirit and scope of the invention in its broadest
form.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention,
reference is now made to the following descriptions taken in
conjunction with the accompanying drawings, in which:
FIG. 1 illustrates a cross-sectional view of a stress determination
circuit for empirically determining shrinkage stress constructed
according to the principles of the present invention;
FIG. 2 illustrates a graphical representation of a stress
dependence of a complex permeability of the toroidal ferrite of
FIG. 1; and
FIG. 3 illustrates a schematic diagram of a power supply employing
a sensor constructed according to the principles of the present
invention.
DETAILED DESCRIPTION
Referring initially to FIG. 1, illustrated is a cross-sectional
view of a stress determination circuit 100 for empirically
determining shrinkage stress constructed according to the
principles of the present invention. The stress determination
circuit 100 includes a sensor 110 embedded within a molded package
170 by a molding compound 180. The stress determination circuit 100
further includes a measurement circuit 190, coupled to the sensor
110 that determines a complex permeability (having real and
imaginary permeability components) under stress of the sensor 110.
The sensor 110, in the illustrated embodiment, consists of a
toroidal ferrite core 120, having a primary (drive) winding 130 and
a secondary (sense) winding 140 through its center hole.
In the illustrated embodiment, the toroidal ferrite core 120 is
composed of MnZn ferrite having an initial real permeability in the
range of 1000 to 3000. Although the illustrated embodiment uses a
MnZn toroidal ferrite core 120, those skilled in the art should
realize that the use of any magnetostrictive material to measure
shrinkage stress falls within the broad scope of the present
invention.
Turning now to FIG. 2, illustrated is a graphical representation of
a stress dependence of a complex permeability of the toroidal
ferrite core 120 of FIG. 1. Complex permeability is a combination
of two components: a real permeability .mu.r' and an imaginary
permeability .mu.r". The real permeability .mu.r' decreases
monotonically as compression stresses are applied to the toroidal
ferrite core 120. In contrast, the imaginary permeability .mu.r"
increases as compression stresses are applied. The compression
stresses on the toroidal ferrite core 120 may therefore be
determined by measuring the real and imaginary permeability .mu.r',
.mu.r".
With continuing reference to FIG. 1, the stress determination
circuit 100 operates as follows. Before molding, the toroidal
ferrite core 120 is calibrated by measuring its unstressed complex
permeability in a control environment (which is preferably
relatively stress-free and most preferably free air).
In one embodiment of the present invention, the measurement circuit
190 applies a drive signal to the toroidal ferrite core 120,
measures a response signal from the toroidal ferrite core 120, and
uses the drive signal and the measured response signal to determine
the complex permeability of the toroidal ferrite core 120.
A preferred embodiment of the present invention uses a conventional
impedance measurement technique to determine the complex
permeability of the toroidal ferrite core 120. The measurement
circuit 190 produces, using conventional processes, a drive voltage
of a known magnitude and phase. The drive voltage is then applied
to the primary (drive) winding 130. The measurement circuit 190
then measures, using conventional processes, a magnitude and phase
of a resulting sensed current generated by the secondary (sense)
winding 140 of the sensor 110. The measurement circuit 190 can thus
compare the drive voltage to the sensed current to determine the
complex permeability of the toroidal ferrite core 120.
In a preferred embodiment, the measurement circuit 190 may use
analog to digital converters to convert the magnitude and phase of
both the drive and the sensed signals to digital signals. The
complex permeability of the toroidal ferrite core 120 may then be
computed. Of course, the measurement circuit 190 may also be
performed by analog circuitry.
Alternatively, the complex permeability of the toroidal ferrite
core 120 may be measured using conventional B-H loop measurements.
B-H loop measurements are familiar to those skilled in the art,
and, as a result, will not be described.
The measurement circuit 190 may, in one embodiment of the present
invention, be located in the molded package 170. Of course, the
measurement circuit 190 may also be located outside of the molded
package 170.
During the three molding steps of filling, packing and cooling, the
sensor 110 may be measured again to determine empirically the
shrinkage stresses imposed on the toroidal ferrite core 120. Again,
conventional methods for determining the complex permeability
(e.g., impedance measurement, B-H loop measurement) may be
used.
In a preferred embodiment, the measurement circuit 190 again
applies the drive voltage to the primary (drive) winding 130. The
molecular magnetic domains within the toroidal ferrite core 120,
however, are restricted by the shrinkage stresses and therefore
cannot achieve the required alignment. The measurement circuit 190
may then measure the magnitude and phase of the sensed current
through the toroidal ferrite core 120 at the secondary (sense)
winding 140. The drive voltage may then be compared to the sensed
current to determine the complex permeability under stress of the
toroidal ferrite core 120. The magnitude of the molding stresses
imposed by the molding compound 180 in the vicinity of the sensor
110 may thus be derived. Those skilled in the art should realize,
of course, that the operability of the sensor 110 and measurement
circuit 190 during the molding of the molded package 170 is not
necessary to the broad scope of the present invention.
Turning now to FIG. 3, illustrated is a schematic diagram of a
power module 300 employing a sensor constructed according to the
principles of the present invention. The power module 300 includes
a power train having a conversion stage (not separately referenced)
including a power switching device 320 for receiving input
electrical power V.sub.IN and producing therefrom switched
electrical power. The power module 300 further includes a filter
stage (not separately referenced, but including an output inductor
330 and output capacitor 340) for filtering the switched electrical
power to produce output electrical power (represented as a voltage
V.sub.OUT).
The power module 300 further includes a power transformer 350,
coupled to the conversion stage, having a ferrite core, a primary
(drive) winding 355 and a secondary (sense) winding 360. The power
module 300 still further includes a rectifier (including rectifying
diodes 370, 380) coupled between the conversion stage and the
filter stage. The power module 300 is embedded in a molded package
by a molding compound (not shown).
The power module 300 further includes a stress determination
circuit for empirically determining stress in the molded package.
The stress determination circuit includes a sensor and a
measurement circuit. The sensor may be integrated into the power
train of the power module 300. In the illustrated embodiment of the
present invention, the power transformer 350 may perform the
function of the sensor. The power transformer 350 contains a
ferrite core that is adversely affected by molding stresses
exhibited by the molding compound. Prior to molding, a complex
permeability of the power transformer 350 (consisting, as described
above, of real and imaginary permeability components) may be
characterized in free air. After molding, the complex permeability
of the power transformer 350 may again be measured. As the molding
compound thermally shrinks, imposing stresses on the ferrite core,
the real permeability monotonically decreases. A method of
indirectly measuring the stresses acting on the ferrite core of the
power transformer 350 is thereby provided. Of course, the output
inductor 330, with its ferrite core and the addition of a second
(drive or sense) winding, may also perform the function of the
sensor. Again, the power transformer 350 and power module 300 are
submitted for illustrative purposes only and the use of the sensor
in other devices and applications are well within the broad scope
of the present invention.
For a better understanding of power electronics including power
supplies and conversion technologies, see "Principles of Power
Electronics," by J. G. Kassakian, M. F. Schlecht and G. C.
Verghese, Addison-Wesley (1991). For a better understanding of
magnetic devices and construction techniques therefor, see
"Handbook of Transformer Applications," by William Flanagan, McGraw
Hill Book Co. (1986). The aforementioned references are
incorporated herein by reference.
Although the present invention has been described in detail, those
skilled in the art should understand that they can make various
changes, substitutions and alterations herein without departing
from the spirit and scope of the invention in its broadest
form.
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